pub fn odd_fibnacci_sum(threshold: u32) -> u32 {
    if threshold < 2 {
        // if is 1, then n < 1 => n = 0, so res also is 0
        return 0;
    }

    let mut res = 0;

    let mut m = 0;
    let mut n = 1;

    while n < threshold {
        if n % 2 == 1 {
            res += n;
        }

        let next = m + n;
        m = n;
        n = next;
    }

    res
}

// fibonacci, 经典
// a, b 然后疯狂加
// Flush for 流水线
